Single dose tolerance to morphine was examined in rats as a function of intertest interval. When two doses were administered 6 hours apart, the second dose was as effective as the first. At a 12 hour intertest interval, some tolerance had developed and by 24 hours, strong tolerance was observed. The increase in single dose tolerance with increased interinjection interval was observed following both systemic injections (20 mg/kg) and microinjections into the periaqueductal gray (PAG, 5 micrograms in 1/2 microliter). Analgesia produced by electrical stimulation of the PAG was partially blocked by Naloxone administered either systemically (5 mg/kg) or by microinjection into the stimulation site via a hollow chemitrode (1 micrograms in 1/2 microliter. The Naloxone reversal of stimulation produced analgesia (SPA) was greater at low stimulation intensities and at sites near the dorsal raphe nucleus. Multiple unit activity was recorded from sites within the PAG where electrical stimulation caused analgesia. Increases in multiple unit activity were observed following morphine administration but not saline administration. The increased activity was only partially reversible by Naloxone suggesting that it represented in part a nonspecific effect in the PAG which supported SPA.